Methods for distinguishing welded steel pipes from seamless steel pipes

1. Metallographic method
Metallography is one of the main methods for distinguishing welded steel pipes from seamless steel pipes. High-frequency resistance welded steel pipes do not have welding materials added, so the weld seam in the welded steel pipe is very narrow. If the method of rough grinding and then corrosion is used, the weld seam cannot be seen. Once the high-frequency resistance welded steel pipe is welded and has not been heat treated, the weld structure will be essentially different from the parent material of the steel pipe. At this time, the metallographic method can be used to distinguish welded steel pipes from seamless steel pipes. In the process of distinguishing the two steel pipes, it is necessary to cut a small sample with a length and width of 40mm at the welding point, perform rough grinding, fine grinding, and polishing on it, and then observe the structure under a metallographic microscope. When ferrite and widmanstatten, parent material, and weld zone structure are observed, the welded steel pipe and seamless steel pipe can be accurately distinguished.

2. Corrosion method
In the process of using the corrosion method to distinguish welded steel pipes and seamless steel pipes, the weld of the processed welded steel pipe should be polished. After the polishing is completed, the polishing traces should be seen. Then, the end face is polished with sandpaper at the weld, and the end face is treated with a 5% nitric acid alcohol solution. If an obvious weld appears, it can be proved that the steel pipe is a welded steel pipe. There is no obvious difference in the end face of the seamless steel pipe after corrosion.

Performance of welded steel pipe
Because the welded steel pipe is processed by high-frequency welding, cold rolling, and other processes, it has the following properties.
①Good thermal insulation function. The heat loss of welded steel pipe is relatively small, only 25%, which is not only conducive to transportation but also reduces costs.
② It is waterproof and corrosion-resistant. During the construction process, there is no need to set up a separate trench. The steel pipe can be directly buried underground or underwater, which reduces the construction difficulty of the project.
③ It is impact-resistant. Even in a low temperature environment, the steel pipe will not be damaged, so its performance has certain advantages.

Performance of seamless steel pipes
Since the metal material of seamless steel pipes has a large tensile strength, it has a stronger ability to resist damage, and it has a hollow channel, so it can effectively transport fluids. It is precisely because of its strong transport capacity that the corrosion resistance of seamless steel pipes is higher than that of welded steel pipes, and its stiffness is relatively large. Therefore, the more loads the seamless steel pipe carries, the more it can be widely used in projects with higher construction requirements.

3. Distinguish welded steel pipes and seamless steel pipes according to the process
In the process of distinguishing welded steel pipes and seamless steel pipes according to the process, all welded steel pipes are welded according to processes such as cold rolling and extrusion. In addition, when high-frequency, low-frequency arc welding pipes and resistance welding pipes are used to weld steel pipes, spiral pipe welding, and straight seam pipe welding will be formed, which will form round steel pipes, square steel pipes, oval steel pipes, triangular steel pipes, hexagonal steel pipes, diamond steel pipes, octagonal steel pipes, and even more complex steel pipes. In short, different processes will form steel pipes of different shapes, so that welded steel pipes and seamless steel pipes can be clearly distinguished. However, in the process of distinguishing seamless steel pipes according to the process, they are mainly distinguished according to the hot rolling and cold rolling treatment methods, and seamless steel pipes also have two main forms, namely hot-rolled seamless steel pipes and cold-rolled seamless steel pipes. Hot-rolled seamless steel pipes are formed by perforation, rolling and other processes, especially large-diameter and thick seamless steel pipes are welded by this process; cold-drawn pipes are formed by cold-drawing of tube billets, and the material strength is lower, but their outer and inner surfaces are smooth.

4. Distinguish welded steel pipes and seamless steel pipes by use
Welded steel pipes have higher bending and torsional strength and greater load-bearing capacity, so they are generally widely used in the manufacture of mechanical parts. For example, oil drill pipes, automobile transmission shafts, bicycle frames, and steel scaffolding used in construction are all made of welded steel pipes. However, seamless steel pipes can be used as pipelines for conveying fluids because they have a hollow cross-section and are long strips of steel with no seams around them. For example, they can be used as pipelines for conveying oil, natural gas, coal gas, water, etc. In addition, the bending strength of seamless steel pipes is relatively small, so they are generally widely used in superheated steam pipes for low- and medium-pressure boilers, boiling water pipes, and superheated steam pipes for locomotive boilers. In short, welded steel pipes and seamless steel pipes can be clearly distinguished by the classification of use.

To ensure higher quality of engineering construction in my country, it is necessary to distinguish welded steel pipes from seamless steel pipes, and then use appropriate steel pipes for construction. In the process of distinguishing welded steel pipes from seamless steel pipes, the metallographic method and corrosion method can be used to observe the structure and welds, thereby inspecting the welds and finally accurately distinguishing them. At the same time, the two types of steel pipes can be distinguished according to their performance, purpose, etc., and then reasonable steel pipes can be selected for construction.